84
M. Yamanaka
4.1 Dependency of External Neutron Source
4.1.1 Experimental Settings
4.1.1.1 Uranium-Fueled ADS Core
The critical EE1 core (reference core) was assembled at the KUCA-A core and was
made up of 25 fuel rods surrounded by polyethylene reflectors as shown in Fig.
A2.1. Each fuel rod (1/8
p60EUEU) was composed of highly enriched uranium
(HEU; 2
× 2
and 1/16
thick) and a polyethylene moderator (PE; 2
× 2
and 1/4
thick) as shown in Fig. A2.2. The core was selected for considering the variation of
β eff along the subcritilicality level. For the measurement of subcriticality, protons
accelerated to 100 MeV were injected onto a disk-type tungsten (W) target in order
to generate spallation neutrons. The accelerator was operated in pulsed mode and
the repetition rate of the pulse was 20 Hz. The time width of the pulsed proton
beam was 100 ns. The averaged proton current was 50 pA. The target was located
at (15, H; Fig. A2.1) grid. The diameter and the thickness of the target were 50 mm
and 12 mm, respectively. The subcriticality was measured by the extrapolated area
ratio method [1] without considering the spatial effects. The neutron signals were
obtained with the use of a BF 3 detector inserted diagonally at (10, U; Fig. A2.1) to the
core for the measurements of the subcriticality. For the reference core in the critical
experiment, excess reactivity and control rod worth (C1, C2, and C3) were measured
by the positive period method and the rod drop method, respectively. Experimental
analyses [2] were available to examine the precision of eigenvalue calculations by the
Monte Carlo method and the accuracy of measured subcriticality by the extrapolated
area ratio method. To achieve deep subcriticality, some of the fuel rods “F” (Fig.
A2.1) were substituted for polyethylene reflectors and configured as shown in Fig.
A2.3d, f, and the subcriticality level then ranged between about 1300 and 7500 pcm.
4.1.1.2 Thorium-Fueled ADS Core
In this experiment, different external neutron sources (spallation neutrons by the
injection of 100 MeV protons onto the W target, and 14 MeV neutrons by the injection
of deuteron beams onto the tritium target) were used for considering the variation of
β eff caused by the spectrum of external neutron source in the subcritical estimation.
The subcritical core at k eff 0.85 (Th-HEU-5PE core shown in Figs. A5.3 and A5.4)
was composed of the polyethylene reflectors, fuel rods of thorium (Th; 2
× 2
and
1/8
thick), HEU, and PE moderators, as shown in Fig. A5.7. 14 MeV neutrons
were produced by 0.4 mA deuteron beam, 10 Hz pulsed frequency, and 10 μs pulsed
width. 100 MeV proton beams were injected onto the W target at 50 mm spot size,
10 pA intensity, 20 Hz pulsed frequency, and 100 ns pulsed width. The subcriticality
was measured by the same method as that in uranium-fueled ADS core. Here, the
measured subcriticality could be affected by spatial effects especially in such a deep
M. Yamanaka
4.1 Dependency of External Neutron Source
4.1.1 Experimental Settings
4.1.1.1 Uranium-Fueled ADS Core
The critical EE1 core (reference core) was assembled at the KUCA-A core and was
made up of 25 fuel rods surrounded by polyethylene reflectors as shown in Fig.
A2.1. Each fuel rod (1/8
p60EUEU) was composed of highly enriched uranium
(HEU; 2
× 2
and 1/16
thick) and a polyethylene moderator (PE; 2
× 2
and 1/4
thick) as shown in Fig. A2.2. The core was selected for considering the variation of
β eff along the subcritilicality level. For the measurement of subcriticality, protons
accelerated to 100 MeV were injected onto a disk-type tungsten (W) target in order
to generate spallation neutrons. The accelerator was operated in pulsed mode and
the repetition rate of the pulse was 20 Hz. The time width of the pulsed proton
beam was 100 ns. The averaged proton current was 50 pA. The target was located
at (15, H; Fig. A2.1) grid. The diameter and the thickness of the target were 50 mm
and 12 mm, respectively. The subcriticality was measured by the extrapolated area
ratio method [1] without considering the spatial effects. The neutron signals were
obtained with the use of a BF 3 detector inserted diagonally at (10, U; Fig. A2.1) to the
core for the measurements of the subcriticality. For the reference core in the critical
experiment, excess reactivity and control rod worth (C1, C2, and C3) were measured
by the positive period method and the rod drop method, respectively. Experimental
analyses [2] were available to examine the precision of eigenvalue calculations by the
Monte Carlo method and the accuracy of measured subcriticality by the extrapolated
area ratio method. To achieve deep subcriticality, some of the fuel rods “F” (Fig.
A2.1) were substituted for polyethylene reflectors and configured as shown in Fig.
A2.3d, f, and the subcriticality level then ranged between about 1300 and 7500 pcm.
4.1.1.2 Thorium-Fueled ADS Core
In this experiment, different external neutron sources (spallation neutrons by the
injection of 100 MeV protons onto the W target, and 14 MeV neutrons by the injection
of deuteron beams onto the tritium target) were used for considering the variation of
β eff caused by the spectrum of external neutron source in the subcritical estimation.
The subcritical core at k eff 0.85 (Th-HEU-5PE core shown in Figs. A5.3 and A5.4)
was composed of the polyethylene reflectors, fuel rods of thorium (Th; 2
× 2
and
1/8
thick), HEU, and PE moderators, as shown in Fig. A5.7. 14 MeV neutrons
were produced by 0.4 mA deuteron beam, 10 Hz pulsed frequency, and 10 μs pulsed
width. 100 MeV proton beams were injected onto the W target at 50 mm spot size,
10 pA intensity, 20 Hz pulsed frequency, and 100 ns pulsed width. The subcriticality
was measured by the same method as that in uranium-fueled ADS core. Here, the
measured subcriticality could be affected by spatial effects especially in such a deep